1. Field of the Invention
The present invention relates to an AC servo drive without current sensor, especially to an AC servo drive without current sensor using open loop circuit control and using current command received by encoder in servomotor as feedback current signal, thus being used in linear servo motor operation.
2. Description of Prior Art
FIG. 1 shows a prior art AC servo module comprising an AC servo driver 1 connected to a servomotor 2. The servomotor 2 comprises an encoder (not shown). The AC servo driver 1 comprises a first PI (proportional integration) controller 11, a second and a third PI controllers 12, 13 for d and q axes, a first coordinate converter 14, a pulse width modulator 15, a plurality of current sensors 16 with three phase connection, a second coordinate converter 17, a decoupling compensator 18, a counter 19 and a speed estimator 20, wherein the coil transformation function (s)=1/(Ls+R), where Ls indicates an inductance for the coil and R indicates resistance for the coil.
In above-mentioned AC servo driver 1, the decoupling has relationship only with the magnetization current Id, the torque current Iq and angular speed w. The decoupling compensator 18 keeps in action and the stator system can be simplified as FIG. 2. The relationship for the second and the third PI controllers 12, 13 for d and q axes, the coil transformation function (s)=1/(Ls+R) for the servomotor 2 is shown. The servomotor 2 obtains the feedback for the stator current, the magnetization current Id, the torque current Iq and uses decoupling current instruction signal Id* and Iq* as the input current command signal for the second and the third PI controllers 12, 13 of d and q axes.
FIG. 3 shows the frequency response of the prior art AC servo module in FIG. 1 and a 3 KW, 2000 rpm motor is used to drive a load of five times initia. The curve A in upper portion of FIG. 3 is a speed command and the curve B is actual output speed. The curve C in lower portion of FIG. 3 is Iq command and the curve D is feedback current. Because the current sensor will sense the feedback current, the curves C and D will be very close.
In above-mentioned prior art AC servo module, the servomotor 2 receives current command signal through the encoder thereof and provides an initial angle for rotator such that the AC servo driver 1 generates a stator current vertical to the rotator magnetic field. The input voltage, the current and rotation speed of the permanent magnetic servomotor 2 are nonlinear, the sensors 18 detect the three-phase current of the servomotor 2 and feedback the three-phase current for decoupling and providing linearization. The feedback scheme for decoupling is important in advanced AC servo architecture. The conventional current sensor generally adopts Hall-effect element to detect stator current in high power AC servo device. However, the Hall-effect element has drift property with temperature, namely, the measurement is increased as temperature is increased. Moreover, a plurality of current sensors is needed for obtaining three-phase current. The feedback current is converted through the second coordinate converter 17. Therefore, it is difficult to cost down in the manufacture and maintenance of this device.